1. The area of deep cardiac dulness, quadrate in form, corresponds to the complete area already mapped out as representing the projection of the heart on to the anterior aspect of the chest wall. (Fig. XV, 1-4.)
2. The area of superficial cardiac dulness, more or less triangular in shape, corresponds to that part of the heart which is not covered by the thin anterior margin of the lung. This area can, (Fig. XVI, 10.) Fig. XV with sufficient accuracy, be defined as a triangular space, the left border being formed by a line from the fourth left chondro-sternal articulation to the apex beat of the heart in the fifth left interspace, the right border by a line which passes downwards along the middle of the sternum from the level of the anterior extremities of the fourth to the seventh costal cartilages, and the base by a line which passes outwards from the level of the seventh costal cartilage to the position of the apex beat.
A reference to Figs. XIV. and XV. will make it evident that paracentesis of the pericardium can be performed, without injury to pleura or lung, in the fifth left intercostal space. The internal mammary artery runs vertically downwards about 1/2 inch from the outer border of the sternum, and the needle should, therefore, be inserted through the fifth intercostal space about 1 inch from the outer border of the sternum.